U.S. patent application number 13/499540 was filed with the patent office on 2012-07-26 for safety device for an electric power steering system.
This patent application is currently assigned to ThyssenKrupp Presta AG. Invention is credited to Imre Benyo, Imre Szepessy, Adam Varga.
Application Number | 20120191301 13/499540 |
Document ID | / |
Family ID | 43014175 |
Filed Date | 2012-07-26 |
United States Patent
Application |
20120191301 |
Kind Code |
A1 |
Benyo; Imre ; et
al. |
July 26, 2012 |
SAFETY DEVICE FOR AN ELECTRIC POWER STEERING SYSTEM
Abstract
The invention relates to a control method for a steering system
having electric power assistance, comprising a control means, for
example a steering wheel, which can be controlled by a driver, an
electric power assist motor, an electric control unit, which
contains a memory for storing digital data, a driver unit (motor
controller), which determines electrical signals for controlling
the power-assistance motor in accordance with a target engine
torque that was transmitted to the driver unit and outputs said
electrical signals to the power-assistance motor, at least one
sensor device for determining a control variable, for example a
manual torque, introduced into the control means, wherein a preset
value for a engine torque of the power-assistance motor is
determined in the control unit with the aid of the control
variable, wherein in addition, an upper threshold value for the
target engine torque is stored in a limiting element, and for a
case A in which the preset value exceeds the upper threshold value,
the limiting element outputs the upper threshold value as a target
engine torque to the motor driver unit, and for a case B in which
the preset value does not exceed the upper threshold value, the
limiting element outputs the preset value as a target engine torque
to the driver unit.
Inventors: |
Benyo; Imre; (Budapest,
HU) ; Szepessy; Imre; (Budapest, HU) ; Varga;
Adam; (Budapest, HU) |
Assignee: |
ThyssenKrupp Presta AG
Eschen
LI
|
Family ID: |
43014175 |
Appl. No.: |
13/499540 |
Filed: |
September 14, 2010 |
PCT Filed: |
September 14, 2010 |
PCT NO: |
PCT/EP2010/005615 |
371 Date: |
March 30, 2012 |
Current U.S.
Class: |
701/41 |
Current CPC
Class: |
B62D 5/0463 20130101;
B62D 5/0481 20130101 |
Class at
Publication: |
701/41 |
International
Class: |
B62D 6/08 20060101
B62D006/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 2009 |
DE |
10 2009 048 092.7 |
Claims
1. A control method for a steering system with electric power
assistance, the steering system including a control means
controllable by a driver, an electric power assist motor, an
electric control unit, which includes a memory for storing digital
data, a motor controller that based on a target engine torque, sent
to the motor controller, determines and sends out electrical
signals for controlling the electric power assist motor, at least
one sensor device for determining a control variable introduced
into the control means, and the method comprising: determining, in
the control unit, with the help of the control variable, a preset
value for an engine torque of the electric power assist motor; and
using an upper threshold value and an upper intermediate value for
the target engine torque stored in a limiting element, for a case
A, in which the preset value exceeds the upper threshold value,
delivering, by the limiting element, the upper threshold value as
the target engine torque directly or indirectly to the motor
controller, for a case B, in which the preset value does not exceed
the upper intermediate value, delivering, by the limiting element,
the preset value as the target engine torque directly or indirectly
to the motor controller, and for a case C, in which the preset
value takes a value that is between the upper intermediate value
and the upper threshold value, determining the target engine torque
by subtracting from the preset value a correction value which is
calculated from a difference between the preset value and the upper
threshold value, and delivering the determined target engine torque
directly or indirectly to the motor controller.
2. The control method according to claim 1, wherein a lower
threshold value for the target engine torque is further stored in
the limiting element, wherein the lower threshold value has a value
that is lower than the upper threshold value, and wherein the
method further comprises: for a case D, in which the preset value
falls below the lower threshold value, outputting, by the limiting
element, the lower threshold value as the target engine torque to
the motor controller, and for a case E, in which the preset value
does not fall below the lower threshold value and does not exceed
the upper threshold value, outputting, by the limiting element, the
preset value as the target engine torque to the motor controller,
and for a case F, in which the preset value takes a value that is
between the lower intermediate value and the lower threshold value,
determining the target engine torque by adding to the preset value
a correction value which is calculated from a distance between the
preset value and the lower threshold value, and delivering the
determined target engine torque directly or indirectly to the motor
controller.
3. The control method according to claim 1, wherein the upper
threshold value is dependent upon the control variable.
4. The control method according to claim 1, wherein the upper
threshold value is dependent upon vehicle speed and/or one or more
other vehicle parameters selected from the group consisting of
steering angle speed, steering angle, available power supply or yaw
rate.
5. The control method according to claim 2, wherein at high vehicle
speeds, the distance between the upper threshold value and the
lower threshold value is smaller than at lower vehicle speeds.
6. The control method according to claim 2, further comprising:
measuring, during a preset length of time, a frequency of reaching
the lower and upper threshold values is measured; and performing at
least one of reducing the upper threshold value or increasing the
lower threshold value, if within the preset length of time, the
upper threshold value and/or the lower threshold value is reached
more than a preset number of times.
7. The control method according to claim 6, wherein the upper
threshold value and/or the lower threshold value is reset to a
respective original value if, for a second preset length of time,
the upper and/or the lower threshold value is no longer
reached.
8. The control method according to claim 2, wherein an
approximation to the lower threshold value is proportional to a
square or logarithmic function.
9. The control method according to claim 2, wherein an
approximation to the lower or upper threshold value is controlled
by means of a PD controller, which uses a distance of the preset
value from the respective threshold value and a change in this
distance as control inputs.
10. The control method according to claim 1, wherein a distance
between the preset value and a respective threshold value is
introduced with a weighting factor into a transition function for
determining the target engine torque.
11. The control method according to claim 10, further comprising
calculating a steering angle speed, wherein the steering angle
speed is introduced along with a weighting factor into the
transition function.
12. The control method according to claim 2, wherein a difference
between the upper intermediate value and the upper threshold value
and/or between the lower intermediate value and the lower threshold
value is dependent upon vehicle speed and/or at least one other
vehicle parameter.
13. The control method according to claim 1, further comprising:
combining an output value for the target engine torque with an
output signal of a stabilisation and an output signal of a damping;
and delivering a result of said combining to the motor controller
as a preset signal.
14. The control method according to claim 2, wherein the lower
threshold value is dependent upon the control variable.
15. The control method according to claim 2, wherein the lower
threshold value-is dependent upon vehicle speed and/or one or more
other vehicle parameters selected from the group consisting of
steering angle speed, steering angle, available power supply or yaw
rate.
Description
[0001] The present invention relates to a control method for a
steering system with electric power assistance having the features
of the preamble of claim 1.
[0002] Motor vehicles with electric power steering generally
comprise a steering column which is connected via steering gear
with the steered wheels of the vehicle. The steering column
contains a torque sensor for the torque that the driver introduces
into the steering. An electric servomotor is also provided, which
drives the steering gear via a reduction gear and assists the
driver in performing the steering. A control is necessary in order
to ensure that the servomotor generates precisely the amount of
power assistance necessary to achieve a certain steering
characteristic. For example, at low speeds and high torques a high
level of power assistance should be generated in order to take the
burden off of the driver when parking and at higher speeds and
lower torques a low level of power assistance should be generated
in order that the driver experiences a direct steering feel. A very
important aspect is that malfunctions of the sensor, the control
system or the electric motor do not lead to the electric motor
performing undesired and unexpected steering manoeuvres.
[0003] The general object of control systems therefore is to
provide interference-free functioning of the electric power
steering. German patent specification DE 100 636 05 B4 provides
that an electric motor is controlled via a driver. In addition a
motor driver limiting device is provided in order to limit the
driving of the electric motor. The driver limiting device switches
off completely if a fault is detected in the motor driver. During
vehicle operation this results in a total and sudden loss of the
power assistance. This can have an irritating effect for the
driver.
[0004] The German published application DE 198 21 220 A1 provides
that the motor current is limited by an upper threshold value. In
this way excess power assistance can be prevented. This limit is
determined on the basis of the back electromotive force. Thus it is
not possible, however, to compensate for instabilities within the
controller itself. Instabilities can be attributed to various
causes. The driver for example may unnecessarily turn the steering
wheel back and forth. The road surface may be uneven, introducing
periodic disturbances into the control system. The steered wheels
of the motor vehicle may have an imbalance, likewise generating
periodic interferences. Such instabilities cannot be compensated by
limiting the motor current. The publication does not provide for
any lower limit of the motor current either, so that the steering
assistance torque can tend to zero. In the case described this
corresponds to a complete and sudden loss of the power
assistance.
[0005] A similar solution is described in U.S. Pat. No. 6,404,156
B1. Here limitation of the power assistance is brought about by
upper and lower threshold values for the motor current. In the
chain of electronic control, comprising the various sensors (torque
sensors, speed sensor), a booster with phase compensation, a motor
driver and the servomotor itself, at the booster and phase
compensation stage the sensor values are processed without any
preset restriction and delivered to the driver. The driver limits
the range of values of the control signal for driving the electric
motor in order to prevent excessively high and low motor currents
and thus excessively high or low steering assistance torques.
[0006] Steering systems according to the prior art described have
the following restrictions on the driving dynamics:
[0007] The steering systems according to DE 100 636 05 B4 and DE
198 21 220 A1 limit the range of values for the possible motor
current in certain driving situations. In this way the maximum
possible motor output and thus the maximum power assistance are
also limited. In extreme situations such as for example evasive
manoeuvres or also extreme and unforeseeable influences on the
steered wheels this can lead to a higher manual torque being
exerted on the steering wheel than is actually necessary on the
basis of the driving situation and the technically available output
of the servomotor. In many situations, therefore, steering systems
do not fully utilise the dynamic range of the servo unit.
[0008] According to a further exemplary embodiment of the steering
equipment according to U.S. Pat. No. 6,404,156 B1 the sensor signal
that is delivered by the torque sensor of the steering equipment to
the control system is limited as a function of certain parameters.
As a result of this, information on extreme values of the torque
sensor, which for example can occur if the driver operates the
steering wheel with a very high manual torque (evasive manoeuvre)
or if extreme influences act upon the steering (potholes, hitting
the kerb, sudden tyre defect), is lost. On the basis of the
previously limited sensor signal the control system is unable to
recognise such situations and therefore cannot respond
appropriately to them. An appropriate response in the stated cases
would be to increase the steering assistance torque to the
technically possible maximum value, in order to keep the manual
torque on the steering wheel within predefined limits. If the
sensor signal is limited before it reaches the control system this
is not possible. As a result neither does this steering equipment
utilise the full dynamic range of the servo drive that is
technically available.
[0009] The object of the present invention is therefore to provide
a control method for electric power steering, which even in
critical steering situations maintains stable driving and increases
fault tolerance. In particular a control for electric power
steering is to be provided which is able to fully utilise the
available dynamic range of the servo drive and which is insensitive
to system oscillations.
[0010] This object is achieved by a control method with the
features of claim 1. The other claims describe advantageous further
development of the invention.
[0011] In a control method for a steering system with electric
power assistance, comprising: [0012] a control means, for example a
steering wheel, controllable by a driver, [0013] an electric power
assist motor, [0014] an electric control unit, which includes a
memory for storing digital data, [0015] a motor driver unit (motor
controller) that based on a target engine torque, sent to the motor
driver unit, determines and delivers electrical signals for
controlling the electric power assist motor, [0016] at least one
sensor device for determining a control variable introduced into
the control means, for example a manual torque, [0017] wherein in
the control unit with the help of the control variable a preset
value for a engine torque of the electric power assist motor is
determined, [0018] it is also provided that [0019] in a limiting
element an upper threshold value for the target engine torque is
stored, and [0020] for a case A, in which the preset value exceeds
the upper threshold value, the limiting element delivers the upper
threshold value as the target engine torque to the driver unit, and
[0021] for a case B, in which the preset value does not exceed the
upper threshold value, the limiting element delivers the preset
value as the target engine torque to the motor driver unit.
[0022] This allows both the sensor signal to be evaluated across
its full range of values and the motor driver unit to impinge upon
the motor with its full available output current so that in extreme
situations the maximum available dynamics of the steering system
can be utilised. The limiting element is arranged in the signal
path between the controller, which determines the preset value for
the target engine torque, and the motor controller. The limiting
element can however also be physically combined with the control
system in a single unit. Here it is immaterial whether the
limitation is achieved purely by software engineering or purely by
hardware engineering or as a combination of software and hardware
engineering.
[0023] If it is also provided that [0024] in the memory a lower
threshold value for the target engine torque is stored, the value
of which is lower than the upper threshold value, and [0025] for a
case D, in which the preset value falls below the lower threshold
value, the limiting element delivers the lower threshold value as
the target engine torque to the driver unit, and [0026] for a case
E, in which the preset value does not fall below the lower
threshold value and does not exceed the upper threshold value, the
limiting element delivers the preset value as the target engine
torque to the motor driver unit, it can also be prevented that the
target engine torque because of incorrect information, for example
from the sensors, causes the power assistance being lost suddenly
and unexpectedly for the driver.
[0027] If an upper intermediate value and a lower intermediate
value are defined, wherein the upper intermediate value is smaller
than the upper threshold value and the lower intermediate value is
greater than the lower threshold value, and in an area between the
upper intermediate value and the upper threshold value the target
engine torque is determined from the difference between the preset
value and the threshold value, a damped approximation of the target
engine torque to the upper threshold value can be achieved. The
same applies for the approximation to the lower threshold value, if
in an area between the lower intermediate value and the lower
threshold value the target engine torque is determined from the
difference between the preset value and the lower threshold value.
This reduces the tendency to oscillation of the controller. This
continuous transition (=damped transition) from the unlimited to
the preset value limited by the upper and/or lower threshold value
for the engine torque ensures that the driver, upon encountering
critical states, experiences a slower deterioration of the
assistance function and can adjust intuitively to this. Such a
continuous transition is when travelling on poor quality roads also
in particular advantageous, however, in steering systems without a
steering wheel, such as those which use control sticks or
joysticks.
[0028] Accordingly, for a case C, in which the preset value
(T.sub.RM) takes a value that is between the upper intermediate
value (max1) and the upper threshold value (max), it is provided
that as the target engine torque a value is determined which
results from the preset value minus a correction value which is
calculated from the distance between the preset value and the upper
threshold value, and that this value is delivered directly or
indirectly to the motor controller (25).
[0029] For a further case F, in which the preset value (T.sub.RM)
takes a value that is between the lower intermediate value (min1)
and the lower threshold value (min), it is provided that as the
target engine torque a value is determined which results from the
preset value plus a correction value which is calculated from the
distance between the preset value and the lower threshold value,
and that this value is delivered directly or indirectly to the
motor controller (25).
[0030] The approximation to the threshold value can take place
proportionally to the square or logarithmically in order to achieve
constant and preferably constantly differentiable transitions. Here
the approximation to the threshold value can also be regulated via
a PD controller, which uses the distance of the preset value from
the respective threshold value and the change in this distance for
the control. It can further be provided that the distance between
the preset value and the respective adjacent threshold value is
introduced with a weighting factor into a transition function in
order to determine the target engine torque. A steering angle speed
can also be determined and introduced with a weighting factor into
the transition function. Both result in an adaptation of the
approximation to the threshold values with damping as a function of
the driving condition.
[0031] This applies in particular if the difference between the
upper intermediate value and the upper threshold value and/or
between the lower intermediate value and the lower threshold value
is dependent upon the vehicle speed and/or other vehicle
parameters.
[0032] The limiting values for the maximum and minimum target
engine torque can be designed to be variable and can thus be
matched to the parameters of the driving situation, if the upper
and/or the lower threshold value is dependent upon the control
variable introduced. In particular the threshold values can be
dependent upon the vehicle speed and/or other vehicle parameters,
such as for example, but not exclusively, the steering angle speed,
the steering angle, the available power supply or the yaw rate.
[0033] At higher speeds the control approximates to a control
system which undertakes no or only minor control interventions if
at high vehicle speeds the distance between the upper threshold
value and the lower threshold value is smaller than at low vehicle
speeds.
[0034] Where threshold values are frequently reached this may be an
indication that there are fault conditions in the steering system.
Therefore in an advantageous further development the frequency of
reaching the upper and/or lower threshold values for the preset
value of the engine torque is evaluated. Advantageously the control
processes are limited as a precaution, if within a predefined
length of time the frequency of reaching the upper and/or lower
threshold value reaches or exceeds a certain level. In this case
the upper or the lower threshold value, or both, is or are changed
in such a way that the permissible range between the threshold
values is reduced. This may be necessary, for example, if the
threshold values are not reached as a result of an external
influence or a driver intervention, but because faulty sensor
signals lead to this.
[0035] Furthermore, in the event of temporary interferences, it may
be advantageous if the threshold value(s) is (are) reset to the
original value(s), if during a second preset length of time the
threshold value is no longer reached.
[0036] In the following an exemplary embodiment of the present
invention is described in more detail using the drawing. This shows
as follows:
[0037] FIG. 1: an electric power-assisted steering system in a
perspective view;
[0038] FIG. 2: a range of values for the engine torque as a
function of the torque sensor signal for the steering according to
the invention;
[0039] FIG. 3: a block diagram of the electric power steering;
[0040] FIG. 4: a schematic representation of the steering system as
a whole, with steering wheel, torque sensor, control unit, motor
driver, motor and steering gear with the steered wheels;
[0041] FIG. 5: the programme sequence of the control system of the
electric power steering in the form of a flow diagram;
[0042] FIG. 6: an example of a permitted range of values of the
torque requirement signal T.sub.RA as a function of the torque
sensor signal T.sub.TS with an example of the behaviour of an upper
and lower intermediate value, after which the transition to the
limitation is initiated;
[0043] FIG. 7: a further example of a permitted range of values of
the torque requirement signal T.sub.RA as a function of the torque
sensor signal T.sub.TS with a behaviour of an upper and lower
intermediate value, that differs from FIG. 7, after which the
transition to the limitation is initiated;
[0044] FIG. 8: a representation of the behaviour of T''.sub.RM
within the permitted range of values with damped transition;
and
[0045] FIG. 9: a representation of the behaviour of T''.sub.RM
within the permitted range of values without damped transition;
[0046] FIG. 10: a representation of the permitted range of values
with a possible restriction of this range of values.
[0047] FIG. 1 shows a motor vehicle power steering system with a
steering gear 1, in which a steering rack is arranged in the
longitudinal direction of the steering gear 1 in a movable manner.
The steering rack carries two track rods 2, which are connected by
means of ball-and-socket joints with the steering rack. The
ball-and-socket joints are arranged in bellows 3 encapsulated
against environmental influences. The track rods 2 are for their
part connected with steering knuckles of the steered wheels. A
displacement of the steering rack in the steering gear 1 thus leads
in a known manner to a pivoting of the steered wheels and thus to a
steering manoeuvre of the motor vehicle.
[0048] By means of a steering shaft 4 a torque is introduced into
the steering. A torque sensor 5 detects the torque introduced into
the steering shaft 4. In order to provide power assistance for the
steering process and thus to reduce the manual torque to be applied
by the driver a servo drive is incorporated in the steering gear 1.
The servo drive comprises a motor housing 6, a gear housing 7 and a
control system 8. The motor and the gear cannot be seen in this
representation.
[0049] During operation, in prior art fashion the driver operates a
steering wheel 9 which then via the steering shaft 4 and a pinion
brings about a displacement of the steering rack in the steering
gear 1. The torque detected in the torque sensor 5 is monitored and
in order to simplify the steering manoeuvre the servo motor is
impinged upon through the control system 8 with current, in order
to assist the steering movement of the driver.
[0050] Multiple possibilities exist for controlling and regulating
the power assisted steering. Thus the control system 8 can in the
simplest of manners provide power assistance via the servomotor, in
that the required engine support torque is simply proportional to
the sensor torque determined. In practice power assisted steering
systems are in many cases controlled via operating maps which are
stored in a memory in the form of a table of values or by the
saving of analytical functions. A value range for the result of
such a control is shown in FIG. 2.
[0051] FIG. 2 indicates in a system of coordinates, in the
horizontal, possible values for a torque signal T.sub.TS, which is
indicated by the torque sensor 5 as a function of the torque
introduced into the steering wheel 9. In the vertical axis a
possible engine torque T.sub.MOT is shown, which is requested from
the motor driver on the basis of the torque signal T.sub.TS. An
upper characteristic curve 11 and a lower characteristic curve 12
provide upper and lower threshold values to the signal T.sub.MOT.
The hatched areas above the characteristic curve 11 and below the
characteristic curve 12 are prohibited areas, which the engine
torque T.sub.MOT should not reach. From the characteristic curve 11
the respective maximum value max is determined accordingly, which
must be delivered for the value delivered to the motor controller
for the torque requirement signal T.sub.RA. From the characteristic
curve 12 the respective minimum value min is determined
accordingly, which must be delivered for the value delivered to the
motor controller for the torque requirement signal T.sub.RA. The
area between the characteristic curves 11 and 12 is the permitted
value range in which the motor signal T.sub.moT should be located.
For a given torque signal T.sub.TS the motor signal T.sub.MOT can
take various values. These values can, for example, be dependent
upon the vehicle speed V.
[0052] FIG. 3 is a block diagram of a power steering according to
the invention. In the block diagram the vehicle speed V and the
signal T.sub.TS from the torque sensor 5 provide the input signals
which are introduced into a controller 20. Further input signals
can be introduced at 21, for example the ambient temperature, the
yaw rate or similar. From the input values the controller 20
calculates a signal for the required engine torque T.sub.RM and the
torque signal T.sub.TS is fully available to the controller and can
therefore be evaluated totally. The controller 20 likewise
generates a signal T.sub.RM, which comprises the complete possible
range of values and thus has a maximum possible dynamic scope.
[0053] A limiting element 22 receives as an input signal the
vehicle speed V.sub.O and the required engine torque T.sub.RM. The
limiting element 22 calculates from this, using a table or based on
analytical functions, a maximum value and a minimum value, which
the engine torque must take for the preset parameter values. In
relation to FIG. 2, the limiting element 22 ensures that the
required torque value does not enter the prohibited hatched areas
of the diagram from FIG. 2. The signal limited in this way by the
limiting element 22 is combined with, for example added to, signals
not described in more detail from a damping element 23 and from a
stabilisation element 24. The combination then provides a
requirement signal T.sub.RA for the actual steering assistance
torque required. The signal T.sub.RA is delivered to a motor
controller 25 which finally impinges upon a servomotor 26 with
current. The signals generated are also delivered to a safety
function 27 which in the extreme case can bring about a shutdown of
the power assisted steering.
[0054] In order to achieve the target broad dynamic range of the
power assisted steering it is important here that the signal
T.sub.TS and the output signal of the motor controller 25 can cover
the full available dynamic range, so that the full bandwidth of the
signal T.sub.TS picked up by the torque sensor can be evaluated.
Apart from this, the motor controller, the output value range of
which is not limited, can call upon the maximum possible steering
assistance performance of the servomotor 26. The limitation as a
function of speed or of other parameters of the required power
assistance torque T.sub.RA takes place in the limitation element
22.
[0055] FIG. 4 shows the controlled system of the power steering
according to the invention in schematic view.
[0056] The handwheel 9 is connected by means of the spindle 4 with
the torque sensor 5. The torque signal T.sub.TS enters the unit
shown here as an integrated module, which comprises the controller
20 and the limiting element 22. Furthermore, the vehicle speed V is
supplied to the unit 20, 22. Further signals 21, as described
above, are taken into account by the control system.
[0057] As a function of the input variables the unit 20, 22
provides the torque requirement signal T.sub.RA to the motor
controller or motor driver 25 which in turn impinges upon the
servomotor 26 with current. Via a gear the servomotor 26 drives the
steering rack and thus the steered wheels of the vehicle. The road
has a reaction via the steered wheels on the steering shaft 4. In
the torque sensor 5 therefore not only do torque signals occur
based on an operation of the steering wheel 9, but also based on
the reaction of the road via the wheels on the steering shaft 4. In
particular torques can also occur at the torque sensor 5 if the
steering wheel 9 is not operated or even if the driver lets go of
it. The invention is not limited to a controlled system as shown in
FIG. 4, however. The invention is also applicable in the case of
steer-by-wires, where there is no mechanical intervention by the
steering wheel 9 on the wheels of the motor vehicle. In this case
the monitor, that is to say the steering model calculation unit
(the calculation module 28 explained below) would deliver
corresponding signals to an actuator not shown here, which
counteracts the steering wheel movement with a corresponding
reaction torque.
[0058] In a particularly advantageous further development the
control takes place with an LQG control algorithm, as described in
the lecture entitled "Optimale Regelung einer elektromechanischen
Servolenkung" (Optimum Control of Electromechanically Assisted
Steering) given to the 5th VDI Mechatronik Conference 2003 in Fulda
(7-8 May 2003) by Hermann Henrichfreise, Jurgen Jusseit and Harwin
Niessen.
[0059] In the preferred exemplary embodiment, as shown in FIG. 4,
between the transmission of the pure sensor signals T.sub.TS and V
a further calculation module 28 is incorporated for the
mathematical model of the steering used. The calculation module 28
contains the mathematical model of the steering used and works as a
kind of state monitor. From the data available at the input for the
torque sensor signal T.sub.TS, the vehicle speed V and other
possible input data 21, the calculation module 28 can calculate a
plurality of parameters and "substitute measured values", without
these having to be measured with separate sensors. These data
include, for example, the friction that occurs within the steering
system and which cannot be readily measured. Friction can indeed be
taken into account with a steering system according to the
invention, however.
[0060] In this way measured values and calculated "substitute
measured values" can be supplied to the controller 20 for
calculation of the preset value for the engine torque.
[0061] FIG. 5 illustrates the process sequence in the power
steering according to the invention, which is carried out in order
to calculate the manual control of the servomotor 26.
[0062] The input signals T.sub.TS and V are evaluated in a
controller and in prior art fashion a required engine torque is
calculated from this which is output as the signal T.sub.RM. The
control unit 20 is known from the prior art. It can for example
work according to the principle of the control unit that is
described in European patent specification EP 1 373 051 B1. This
control unit works as described above as a so-called state monitor,
which from input variables calculates various output variables and
internally used data. In the known control unit, which can
correspond to the control unit 20, the mathematical model of the
steering is stored, which contains the various dependencies between
the measured values and the non-measured state values. It can,
however, be provided that the control unit 20 takes the form of a
relatively simple control unit in the form of a PID controller or
similar.
[0063] The engine torque signal T.sub.RM is then passed to the
already mentioned components, namely the damping part 23 and the
stabilisation part 24. In parallel the limiting element 22 also
receives this signal. The further input signal, the vehicle speed
V, similarly goes to the limiting element 22 which is shown here as
a broken line.
[0064] In the limiting element 22 in a calculation step 30 from a
table or using analytical functions the permitted threshold values
(max upper threshold value and min lower threshold value) of the
engine torque requirement signal T.sub.RM are now calculated. The
actual signal T.sub.RM delivered by the control unit 20 is then
compared in a first step 31a with the upper intermediate value
max1. If this value max1 is reached a damping of the value T.sub.RM
takes place accordingly, as described in the following. Otherwise
the value T.sub.RM remains unchanged. The result is delivered as
T.sub.RM to the next step. In the second step 31 the value T.sub.RM
is compared with the upper threshold value max. If T.sub.RM is
greater than max, then T'.sub.RM=max is set. If T.sub.RM is smaller
than the threshold value max, then T'.sub.RM=T.sub.RM remains
unchanged. This is illustrated in calculation steps 32 and 33. The
signal with this upper threshold value is delivered to step 34a, in
which the signal T.sub.RM is compared with the lower intermediate
value min1. If T'.sub.RM is smaller than the lower intermediate
value min1, then a corresponding damping of this value T'.sub.RM
takes place, as described in the following. Otherwise the value
T'.sub.RM remains unchanged. The result is passed as T'.sub.RM the
next step 34, in which the signal T.sub.RM is compared with the
lower threshold value. If T'.sub.RM is smaller than the lower
threshold value min, then T'.sub.RM is replaced by min. This takes
place in step 35.
[0065] If it is found in step 34 that T'.sub.RM is not smaller than
min, then T''.sub.RM=T'.sub.RM is output unaltered.
[0066] If in calculation steps 31 or 34 it is found that a
threshold value max or min has been reached, this information is
passed on to a correction element 36. The correction element 36
checks how often the threshold value max or min has been reached or
exceeded. Depending on how it is programmed for the respective
steering system the correction element 36 can then calculate new
threshold values max and min and calculate corresponding new upper
and lower intermediate values max1 and min1, which deviate from the
original threshold values or intermediate values. These new
threshold values and intermediate values are then used for future
calculations in calculation step 30. For example, in the event of
frequent exceeding of a threshold value it may be the case that the
torque sensor 5 is defective and is delivering torque values
T.sub.TS that are too high, too low or which oscillate. In this
case the correction element 36 can provide that the threshold
values max and min are approximated to one another, so that the
output signal T''.sub.RM of the limitation element 22 is further
limited with regard to the possible range of values. Oscillations
in the input signals are then delivered to the motor controller
only to a limited extent.
[0067] The correction element 36 is also programmed in such a way
that in the absence of threshold values being exceeded the
threshold values max and min are reset to the original values. In
practice the correction element 36 can be programmed in such a way
that a narrowing of the threshold values takes place if within five
seconds a plurality of threshold value exceedances are detected.
Resetting of the threshold values then takes place if the
previously narrowed threshold values are no longer reached or
exceeded for a preferably greater length of time, for example 40
seconds. In this way the response of the correction element 36 to
temporary interference does not have a lasting effect on the
behaviour of the steering system.
[0068] As the output signal of the limiting element 22 therefore a
signal is generated which represents the unchanged signal
T''.sub.RM=T.sub.RM if in particular in steps 31 and 34 it is found
that T.sub.RM is smaller than max and greater than min and in steps
31a and 34a it is found that T.sub.RM is smaller than max1 and
greater than min1. If in steps 31 or 34 the threshold values are
exceeded upwards or downwards, then the respective current
threshold value is delivered as an output of the limiting element
22.
[0069] This output signal T''.sub.RM is delivered to an adder 37,
which also contains the output values of the damping element 23
(which should not be confused with the damping which itself takes
place in the limiting element in steps 31a and 34a) and of the
stabilisation element 24. The latter can have positive or negative
signs and are combined in the adder 37 to make a torque requirement
signal T.sub.RA. The signal T.sub.RA is then delivered to the motor
controller 25, which energises the servomotor 26 accordingly.
[0070] It is nevertheless conceivable and possible to replace the
addition of the output signals from the damping 23, stabilisation
24 and limitation 22 by another combination. For example, a
multiplication or more complex function could be used for the
combination.
[0071] It is also conceivable and possible for the output signal
T''.sub.RM to be delivered by the limitation 22 directly to the
motor controller 25 as a torque requirement signal T.sub.RA. In
certain cases through the damped transition from calculated preset
value for the engine torque signal T.sub.RM to the limited actual
output signal T''.sub.RM a sufficient stability and safety for the
steering system can be provided and the additional damping and
stabilisation functions can be dispensed with.
[0072] It should be stressed that the output signal
T''.sub.RM=T.sub.RM of the control unit 20 remains unchanged by the
limiting element 22, provided that the threshold value and
intermediate values max, max1 and min1, calculated in step 30 are
not exceeded. Thus for the signal path T.sub.TS to T.sub.RA the
entire possible dynamics are available.
[0073] The limitation that takes place in steps 33 and 35,
evaluates the full information range of the torque sensor T.sub.TS
and the other input data of the control unit 20. In the case of
limitation also the torque requirement signal T.sub.RA delivered to
the motor controller 25, as a result of the added
damping-stabilisation components, can be greater than or smaller
than the upper limits max and min from the limitation element 22,
so that the motor controller 25 and accordingly the servomotor 26
can develop a higher level of dynamics than could be envisaged
simply on the basis of the limiting element 22.
[0074] FIG. 6 and FIG. 7 provide an illustration of the permitted
range of values as in FIGS. 2 and 10. In FIG. 7 a dot-dash line 41
identifies an upper intermediate value, and dot-dash line 42 a
lower intermediate value. The intermediate values 41 and 42 are
transition values, at which the torque requirement signal T.sub.RA
is not calculated directly from the signal T.sub.RM, and in fact
not even if the signal T.sub.RM is within the limits max and
min.
[0075] With the exemplary embodiment described here a check is made
that the input signals of the limiting element 22 achieves the
intermediate value 41 or 42. If this is the case, the signal
delivered to the motor controller 25 is calculated from the
difference between the intermediate value 41 and the upper
threshold 11 or the difference between the intermediate value 42
and the lower threshold 12. In this way upon approximation to the
threshold values 11 or 12 the signal delivered to the adder 37 is
smaller. In the ideal case an asymptotic approximation to the
threshold values 11 and 12 takes place so that in normal operation
these cannot be exceeded. Strictly speaking therefore in the
particularly preferred case the steps 31 and 32, in which the
comparison with the upper threshold max and lower threshold min
takes place, would be superfluous since in the damping in step 31a
or 34a the damping in the particularly preferred case would take
place in such a way that the transition curve from the preset value
T.sub.RM to the output value T''.sub.RM results in a constantly
differentiable curve, that is to say a curve without jumps, or even
better a curve whose derivation is also without jumps. In the
simplest case a correction value, which is determined by the
difference between the preset value T.sub.RM and the threshold
value, in the case of reaching the upper intermediate value max1 is
subtracted from the preset value T.sub.RM and in the case of
reaching the lower intermediate value min1 is added to the preset
value T.sub.RM. The correction value here can be described by
linear, logarithmic or exponential functions. Here the correction
value preferably has a value of zero in the case that the preset
value T.sub.RM is exactly equal to one of the two intermediate
values max1, max2, and takes a higher value in the case of the
threshold value being reached. It is even conceivable and possible,
where the threshold value is exceeded, to increase the correction
value further, so that the changed preset value never exceeds the
threshold value. This approach is illustrated in more detail in
FIG. 8 below.
[0076] FIG. 8 shows the behaviour of the signal T''RM at the output
of the limiting module 22 for the exemplary embodiment described in
FIG. 7, in which intermediate values 41 and 42 are provided for
when approximating to the threshold values max or min. An
illustration is given of how the value T''.sub.RM upon
approximation to the threshold values max and min does not increase
linearly as far as the threshold values, but from the intermediate
values 41 and 42 is approximated to asymptotically. Such transfer
functions, which can be used for calculating the damping, based on
the exponential or logarithmic function are known and are therefore
not described further here.
[0077] FIG. 9 shows the other case, as has already been described
in connection with FIG. 5. Here, upon reaching the threshold values
max or min, the signal T''.sub.RM comes up against a `hard` stop.
This results in the signal behaviour in a constant, but not
constantly differentiable component which could result in system
oscillations. In the exemplary embodiments of FIGS. 7 and 8 this is
avoided.
[0078] FIG. 10 shows the range of values for the engine torque
(T.sub.MOT) as a function of the torque sensor signal T.sub.TS. The
solid lines 11 and 12 have already been described in FIG. 2. The
dot-dash lines 43 and 44 identify the limited upper threshold value
and the limited lower threshold value following processing by the
correction element 36 in FIG. 5. The permitted range of values for
the engine torque as described under FIG. 5 in the presentation of
the method of working of the correction element 36, is between the
lines 43 and 44. The range of values is consequently further
restricted.
KEY
[0079] 1. Steering gear [0080] 2. Track rod [0081] 3. Bellows
[0082] 4. Steering shaft [0083] 5. Torque sensor [0084] 6. Motor
housing [0085] 7. Gear housing [0086] 8. Control system [0087] 9.
Steering wheel [0088] 11. Upper characteristic curve [0089] 12.
Lower characteristic curve [0090] 20. Controller [0091] 21. Signal
[0092] 22. Limiting element [0093] 23. Damping element [0094] 24.
Stabilisation element [0095] 25. Motor controller [0096] 26.
Servomotor [0097] 27. Safety function [0098] 28. Calculation module
[0099] 30. Calculation step [0100] 31. Calculation step [0101] 32.
Calculation step [0102] 33. Calculation step [0103] 34. Calculation
step [0104] 35. Calculation step [0105] 36. Correction element
[0106] 41. Upper intermediate value [0107] 42. Lower intermediate
value [0108] 43. Limited upper threshold value [0109] 44. Limited
lower threshold value
* * * * *